The ABC of Flash Photography – Terminologies

You will come across few terms which are used in telling you about how to bring the flash to your control. But unless you have the background of these terms (and how a flash works), it will be a guessing game for you (and land up in the… ‘ok just tell me the settings and I will shoot perfectly’ end of the road).

So in this article, I cover few (very) basics of terminologies and flash internal functioning (Just the ‘A’ of ABC). But these fundamentals will make your foundations strong and make you ready for the ‘real field work’.

By the way, most of this terms and explanations are applicable to internal built-in pop-up flashes as well. But if you haven’t read my previous article about the serious downfalls of the pop-up flash, you can read about it here:

I. Internal functioning of a flash

A flash emits burst of light and the amount of light which is emitted is determined by the size of the flash tube and the energy of the electric discharge (compare lightning). The discharge is ‘quenched’ early, if full flash is not necessary.

II. Flash Metering System

The internal electronics inside the camera and the flash that determines how much light is present in the environment (scene) and depending on that how much flash power is required to properly illuminate the “subject”. The automatic flash metering system in the camera determines when the flash discharge needs to be stopped to give the correct exposure.

So you can understand, we are leaving “everything” to the camera and the flash in deciding starting from what is the subject to how much flash power is required. As you have guessed it, things may not be always be correctly estimated and then the photographer’s judgment comes in (or the dreaded M Mode).

II. TTL and ETTL (and the newer ETTL II) Metering Method:

There are more than the above 3 methods of flash metering system available, but I will concentrate only on these as these are the most available and applicable throughout the SLR ranges.

TTL stands for ‘Through The Lens’ flash metering. This mode is used only with film cameras where the light reflected from the “film” is measured by electronic sensors in (or above) the base of the camera. When enough light has been received the flash is cut off. Since we are mostly in D-world, I will skip the detail explanation of this type of metering.

E-TTL stands for ‘Evaluative Through The Lens’ metering. In this mode the flash tube emits a pre-flash and the camera’s automatic metering system is used to determine how much flash power is necessary for proper exposure. This method results in more accurate flash metering as the exposure is determined prior to the main flash firing.

Note: E-TTL mode is required for DSLRs as the digital sensor does not reflect light in the same way as film does and so TTL metering cannot be used.

E-TTL II is similar to E-TTL with the addition of the feature of the camera body returning the focus distance to the flash which can be used in determining flash exposure.

Note: E-TTL II is a camera body based modification of E-TTL, hence all E-TTL capable flashes can also operate in E-TTL II mode if used with an E-TTL II compatible body.

BONUS TIP: The flash metering system works separately (and independently) than the camera’s metering system. This means with already other sources of light present (ambient light), the Av, Tv and P modes adjusts the camera setting appropriately (either by varying shutter, aperture or both respectively) solely based on the ambient light. This is camera’s metering system in work. Whereas, the flash metering system works when the shutter is half pressed and sets the best suitable flash power BASED ON the camera exposure settings.

Note 1: The P mode uses no slower shutter speed than 1/60sec when flash is on.

Bottom-line: The automatic flash metering system DOES NOT change your camera’s exposure settings. Your camera’s metering system DOES NOT take into account the presence of flash when setting the exposure settings.

III. Guide Number (GN)

Guide Number(GN) is a measure of the power of the flash in terms of aperture(A) in f-stop and distance(D) of flash from subject.

GN = A x D

It is normally expressed as x ft/m at ISO y at focal length L.

The GN assumes an undiffused direct path from flash to subject (and not bounce flash or using diffusers – a separate topic altogether).

This value can be used in different ways, one of them is to manually set the correct aperture for proper exposure when the distance D between the flash and the subjects is known provided the same ISO specified in the guide number is used.

If you use 4 times the ISO, then the GN value doubles. Alternatively you can also determine the maximum distance that will get properly illuminated at a given aperture.

IV. Sync Speed

By definition, sync speed = the fastest shutter speed at which the first curtain of the shutter fully opens before the second curtain closes.

Now let me explain this. When you press the shutter, an opaque screen (first curtain) opens up to let the light flow in. Then another opaque screen (second curtain) follows cutting out the light.

Say you have selected shutter speed x sec. So the first curtain will open and the second curtain will close at speeds to give the exposure of x secs. Now if x is too fast, the second curtain will have to start closing even before the first curtain has completely opened. This to allow only x secs of exposure. Thus you can imagine a slit is formed between the first curtain and the second curtain. This situation is absolutely a no-no for flash photography. The first curtain needs to be fully open before the second curtain comes down so that the complete scene is properly lit. For the proper exposure from flash source, we must select the shutter speed slower than x secs (the sync speed) where any shutter speed faster than x sec will cause the second curtain to start closing before the first curtain is completely open.

See the following illustration of what is required for proper exposure by flash:

As mentioned earlier if the shutter speed is faster than the sync speed, the second curtain of the shutter starts closing and actually forms a slit which moves across the image. If the flash is fired during this time, then only a part of the image will be captured by the sensor as lit by the flash (which is quite sad).

Note 1: The faster the shutter speed, the narrower the slit.

Note 2: The sync speed is the property of the camera body.

Note 3: This topic definitely needs a second discussion in details (upcoming articles).

V. FP sync

FP stands for ‘Focal Plane’. If you DO need to use the flash with shutter speed higher than the sync speed (and do not want only a portion of your photo to be lit, the rest being black), you have to use the FP sync mode of your flash (if it supports it). In this mode the flash tube emits a series of very fast flash bursts so that it seems like a continuous light to the camera sensor (or film). This allows the use of fast shutter speeds (see illustration link below). Of course this mode reduces the effective power of the flash (GN) considerably and drains a lot of flash battery power. But, hey you get your job done.

VI. Conclusion

As you can see there’s a LOT of things involved when you are shooting with a flash at different levels, so this topic needs to settle down in your brain and photography as your second nature bit by bit. I have explained the basic terminologies here. If you haven’t got hold of any flash photography books yet to read more in details, I would highly recommend getting one (from library or book store).

Do post comments if you need further clarifications on any terminologies as I would be using them a lot in my subsequent articles on Flash Photography.

About the author
Sudipta Shaw is a Software Professional and a Self-made Photographer. He likes to mentor and teach. He is the founder of the site http://www.pleasurephotography.com

This entry was posted on Thursday, January 21st, 2010 at 3:48 am and is filed under Photography Tips. You can follow any responses to this entry through the RSS 2.0 feed.
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